Maternal salivary estriol (E3) increases prior to preterm labor (McGregor et al, American J Obstet Gyn, Oct 1995). It has been previously reported that plasma E3 levels increase at night. Studies in primates note increased nocturnal E3 levels are linked with uterine contractions and onset of parturition. We evaluated diurnal levels and variations of salivary E3 in women to better understand mechanisms of labor and to validate and characterize saliva E3 testing. Fourteen pregnant women had saliva E3 levels measured hourly from 8:00 am until midnight and every 2 hours from 12:00 am until 18:00 am samples were tested and concentration of E3 was determined using a calibration curve. Results: subjects uniformly demonstrated increased salivary e3 each night (10:00 pm to 6:00 am) versus daytime (8:00 am to 9:00 pm) (p .001) E3 surged at 10:00 pm, peaked at 2:00 - 4:00 am and fell 6:00 - 7:00 am. Conclusion: diurnal variation of salivary E3 (nocturnal rise, morning NADIR) mirrors other steroid hormones as well as uterine activity. E3 may play direct or indirect roles in activation as well as stimulation of human parturition. Clinical use of maternal E3 levels for predicting the onset of preterm labor must account for diurnal variation.